Coal converting process

ABSTRACT

A PROCESS FOR CONVERTING COAL CONTAINING REFUSE TO A USEABLE COAL PRODUCT OF A PREDETERMINED MAXIMUM MOISTURE CONTENT, COMPRSING THE STEPS OF SEPARATING WET, REFUSE-CONTAINING COAL INTO FRACTIONS ACCORDING TO PARTICLE SIZE, INCLUDING A FINEST FRACTION AND ONE OR MORE LARGER FRACTIONS, REMOVING REFUSE AT LEAST FROM THE LARGER FRACTIONS, DEWATERING THE LARGER FRACTION TO A MOISTURE CONTENT BELOW HE PREDETERMINED MAXIMUM, AND MECHANICALLY DEWATERING THE FINEST FRACTION IN A CONTINUOUS CENTRIFUGE CONSTRUCTED TO PROVIDE A FINEST COAT OUTPUT HAVING A MOISTURE CONTENT ABOVE THE PREDETERMINED MAXIMUM WHICH, WHEN THE FRACTIONS ARE ALL RECOMBINED, WILL PROVIDE A COMBINED OISTURE CONTENT WITHIN THE PREDETERMINED MAXIMUM TO FORM SAID PRODUCT.

May 18, 1971 J. N. GERWIG COAL CQNVERTING PROCESS Filed July 9, 1970 United States Patent 3,579,442 COAL CONVERTING PROCESS Jack N. Gerwig, South Charleston, W. Va., assignor t Bird Machine Company, South Walpole, Mass. Filed July 9, 1970, Ser. No. 53,508 Int. Cl. B01d 37/04 U.S. Cl. 210--44 Claims ABSTRACT OF THE DISCLOSURE A process for converting coal containing refuse to a useable coal product of a predetermined maximum moisture content, comprising the steps of separating wet, refuse-containing coal into fractions according to particle sze, including a finest fraction and one or more larger fractions, removing refuse at least from the larger fractions, dewaterng the larger fraction to a moisture content below the predetermined maximum, and mechanically dewatering the finest fraction in a continuous centrifuge constructed to provide a finest coal output having a moisture content above the predetermined maximum which, when the fractions are all recombined, will provide a combined moisture content within the predetermined maximum t0 form said product.

This invention relates to cleaning coal.

In conventonal coal-cleaning procedures, the usual mechanical separation procedures leave a finest fraction, typically particles of 10 mesh and finer, still uncleaned. Conventional separating procedures subsequently applied to this finest fraction to reduce its ash content result in a coal fraction having, on the average, up to 65 or even 75% moisture. T0 dry this fraction adequately for shipment or use, it has been necessary to use elaborate equipmente.g., consecutvely arranged vacnum filters and thermal dryers. Despite the obvious economic disadvantage of having to use such a quantity of equipment to recover this coal, the disadvantages of disposing entirely of this fraction are nearly as greatwhether considered in terms of the potental water pollution resulting from dumping of this coal, or the expensive solidifying equipment required to convert the fines to disposable solid waste.

In addition, the use of thermal dryers creates more problems. Not only can the fine coal particles cause fires in, or be largely lost in the thermal dryers; in addition, hot coal from thermal dryers, unless subjected to a timeconsuming cooling step, can cause hot spots and therefore fires in the storage bins in which the cleaned coal fractions are collected.

It is an object of the present invention to provide eflicent and economical recovery of fine coal particles in coal cleanng processes. Other objects are to reduce the probability of fires in dryers and storage bins and to control dust, while producing a sufliciently dred coal sample for storage and shipment, all in an efiicient, simple, and economical manner.

The inventon features a process for converting coal contaning refuse to a useable coal product of a predetermined maximum moisture content, comprising the steps of separating Wet, refuse-containing coal into fractions according to particle sze, includng a finest fraction and one or more larger fractions, removing refuse at least from the larger fractions, de watering the larger fraction to a moisture content below the predetermined maximum, and mechanically dewaterng the finest fraction in a continuous centrifuge constructed to provide a finest coat output having a moisture content above the predetermined maximum which, when the fractons are all recombined, will provide a combined moisture content within the predetermined maximum.

In preferred embodiments, a finest fraction consisting substantally of 10 mesh and finer particles is dewatered in a screen bowl centrifuge; and, the maximum moisture content is 6%, the finest fraction is dewatered to about 13% or less, the largest fraction to about 3% or less and another fraction, intermediate of the largest and smallest, to about 8% or less, both of the larger fractions being dewatered in oscillating centrifuges.

In one embodiment, the finest fraction has refuse removed therefrom in flotation apparatus, prior to dewatering, and in another embodiment the finest fraction is the underflow from a cyclone separator (preferably, being particles in the range of 200 mesh and larger).

In still another embodiment, the intermediate fraction is thermally dewatered in a thermal dryer. The recombining of this thermally dewatered fraction with the mechanically dewatered finest fraction causes the thermally dewatered coal to be cooled with simultaneous vaporization of moisture from the finest fraction.

Other objects, features, and advantages will appear to one skilled in the art from the following description of preferred embodiments of the present invention, taken together with the attached drawings thereof, in which:

FIG. 1 is a schematic flow sheet of a coal treating process embodying this invention; and,

FIG. 2 is a portion of a flow sheet for another coal treating process embodyng this invention, this portion for replacing the similarly dash line enclosed portion of the process of FIG. 1, with the remainder of this process being otherwise identical to that of FIG. 1.

Referring t0 FIG. 1, raw coal having particles of 1%" and smaller, is mechanically separated by sizing screen 12 at /8" into a large fraction (1%t" x /a") and a smaller fraction (about 30% solids, maximum particle size about The larger fraction passes to a mechanical dryer 14 (e.g., an oscillating centrifuge such as described in U.S. Pat. No. 3,133,879) which provides as underflow a clean, dewatered fraction (about 2-3% moisture) for storage bin 16 (e.g. a railroad car). The smaller fracton, together with efliuent from dryer 14, are passed to a mechanical cleaner 18 (e.g. washing cyclones, Deister tables, F.C. jgs) which substantially removes the larger refuse or ash particles, at least down to below 28 mesh. The cleaned coal fraction from separator 18 is fed to another sizing screen 20, which makes a separation at 28 mesh. The larger particles, a medium fraction, x 28 mesh, pass to a mechanical dryer 24 (e.g., also an oscillatng centrifuge like dryer 14), where it is dewatered to an underflow of about 78% moisture, which is fed to st0rage bin 16. The eflluent from dryer 24 is admixed with efiuent from dryer 14 upstream of cleaner 18.

The finest fraction (28 mesh x 0) from szing screen 20 passes to a conventional froth flotation apparatus 30, wherein the denser and smoother refuse particles (e.g., ash) are separated from the lighter, coarser coal particles. The clean coal slurry from the flotation cells, typically having a moisture content of 6575% passes to a continuous screen bowl centrifuge 34 (e.g. such as described in U.S Pat. No. 3,348,767) wherein the moisture content is reduced sufliciently (e.g., down to 12- 13%) to allow the resultant dewatered fraction to be passed directly to storage bin 16. Centrifuge 34 has two efiluents, of which that from the downstream screen portion is returned to flotation apparatus 30.

Refuse from cleaner 18 passes to refuse dewatering screen 40, and thence to refuse container 42. The effluent from screen 40, together with refuse from froth flotation apparatus 30 and the effluent from the upstream imperforate portion of screen bowl centrifuge 34 all pass to a refuse thickener 48 (e.g., gravity-type clarifier), of which the overflow may be recirculated (e.g., for initial wetting of coal, etc.), and the underflow is sent to a 3 mechanical dewatering apparatus 50 (e.g. a solid bowl centrifuge such as descrbed in U.S. Pat. No. 3228,593). The solids emerging from dewatering apparatus 50 are fed to refuse container 42, whereas efliuent is recirculated to thickener 48.

Per some applications, where particularly dry cleaned coal is desired, one or both the cleaned x 28 mesh and /8" x 1%" fractions exiting mechanical dryers 14 and 24 may be passed to a thermal dryer (indicated in phantom at 52) prior to being loaded into storage bin 16. In such applications, the fine moist coal particles frorn centrifuge 34 cool the thermally dried coal sufliciently to prevent hot spots or fires in the storage bin, while at the Same time being further dried by the hot coal mass from the thermal dryer. Moreover, since the fine particles are not passed to the thermal dryer, they are completely recovered and, additionally, operation of the dryer is unimpeded. The moistness of the fine particles furthermore eliminates dust problerns in the storage bin.

FIG. 2 shows a hydrocyclone cleaner 60 in place of the flotation apparatus 30 of FIG. 1, connected to receive 28 mesh x partcles from sizing sereen 20. The cleaner is constructed to make a separation at about 200 mesh, and pass, as underflow, 28 x 200 mesh coal, together with a tolerably low level of ash particles, to sereen bowl centrifuge 34. The waste overflow or efliuent from cyclone cleaner 60 is passed to thickener 48. The dewatering action of sereen bowl 34 is suflicient to reduce the underflow from cyclone cleaner 60 to a level suitable for admixture with the 1% x and x 28 mesh fractions to produce an acceptably 1ow moisture content in the product of bin 16.

The separation into the finer and coarser fractions is carried out at different mesh sizes in different coal processing operations, 28 mesh being shown as illustrative only. For example, up to about mesh particles can be handled in fiotation cells.

In general, the moisture content of the finished coal product in the storage bin is not greater than about 6%, and hence is suitable for most purposes. This 10W meisture is achieved without thermal drying by use of a continuous centrifuge to reduce the moisture content of the finest fraction to a sufiicently low level that, when that fraction is recombined with the larger fractions, the total mixture has a moisture content of about 6% or less. In addition, as described above, where a thermal dryer is used on the medium and/or largest fraction, the total moisture content of the coal mixture passing to the storage bin may be even further reduced.

Other embodiments will occur to those skilled in the art and are within the following claims,

What is claimed is:

1. In the process for converting coal containing refuse to a useable coal product having a predetermined maximum moisture content, the steps comprising separating wet, refuse-containing coal into fractions according to particle size, including a finest fraction and at least one larger fraction, removing refuse at least from said larger fraction, dewatering said larger fraction to a moisture content below said predetermined moisture content, and

mechanically dewatering said finest fraction in a continous centrifuge to provide a finest coal output of a moisture content above said predeterrnined maximum which, when said fractions are recombined, will provide a combined moisture within said predetermined maximum,

and recombining said dewatered fractions to form said product.

2. The process of claim 1 wherein said continuous centrifuge is of the sereen bowl type.

3. The process of claim 1 wherein said finest fraction consists essentially of particles of 10 mesh and finer.

4. The process of claim 1 wherein said predetermined moisture content is at most about 6%.

5. The process of claim 4 wherein said finest coal fraction is dewatered at least to about 13% moisture.

6. The process of claim 5 Wherein said coal is separated into at least two larger fractions, of which the largest fraction is dewatered to a rnoisture content of at most about 3% 7. The process of claim 6 wherein the smaller fracti0n of said two larger fractions is dewatered to a moisture content of at most about 8%.

8. The process of claim 6 wherein the smaller fraction of said two larger fractions is thermally dewatered in a thermal dryer, the recombining of said thermally dewatered fraction and said finest fraction causing said thermally dewatered coal to be cooled with simultaneous vaporization of moisture from said finest fraction.

9. The process of claim 1 including the step of removing refuse trom said finest fraction in flotation apparatus.

10. The process of claim 1 wherein fractions larger than said finest ractions are each dewatered in an oscillating centrifuge.

References Cited UNITED STATES PATENTS 2,656118 10/1953 Chelminski 20910X J. L. DECESARE, Primary Examiner- U.S. Cl. X.R.

Dedication 3,579,442.Ja0k N. Gerwz'g, Snuth Charleston, W. Va. COAL CONVERT ING PROCESS. Patent dated May 18, 1971. Dedication filed J 111y 1, 1971, by the assignee, Bz'rd Machine Uompcmy. Hereby dedicates to the Public the entire term of sad patent.

[Oficzal Gazette Nmaember 16', 1971.] 

